One objective of space altimetry is to measure the topography of the Earth's surface, that is to say the oceans, the ice caps and the land above sea level. Its principle is illustrated in FIG. 1. A satellite 101 determines on the one hand its precise altitude 111 relative to an Earth-surface reference point 105 (a terrestrial ellipsoid or geoid), by virtue of a precise positioning system (a GPS, DORIS, GLONASS or Galileo system), and on the other hand, the altimetric distance 112 by measuring the time taken by a signal to travel a distance there and back through the atmosphere 104 between the satellite 101 and the Earth's surface 102 (which may be the sea level). Knowing these two items of information 111, 112, the system deduces the height of the Earth's surface relative to the reference surface 105.
Such a satellite 101 is fitted with a payload dedicated to the altimetry measurements as illustrated in FIG. 2. This payload notably comprises the following instruments:                an altimeter radar 201 which measures the altitude between the Earth's surface and the satellite 101;        a system 203 for very precise positioning of the satellite (sometimes designated by the acronym “POD” for “Precise Orbit Determination”) in relation to the reference stations on the ground (the DORIS system) or with reference satellites in orbit (GPS, GLONASS or Galileo systems) in order to be able to reference the topography of the Earth's surface in a terrestrial frame of reference;        a radiometer 205 for correcting the propagation uncertainties due to the water-vapor content of the troposphere.        
For very high-performance payloads, for example for measuring the topography of the oceans, the altimeter operates on two frequencies, typically Ku and C, in order to allow an effective correction of the effect of the ionosphere, that is on a single but very high frequency, usually in the Ka band, in order to reduce the effect of the ionosphere to a negligible level, the residual error being corrected by using models of the ionosphere the precision of which is then sufficient.
The abovementioned instruments usually take the form of instruments that are independent from one another, each comprising notably its own antenna, its own analogue processing modules, its own digital processing modules, its own power supply converters. The instruments thus constitute a bulky and heavy payload.
Techniques have been used to share the antenna between the altimeter and the radiometer. However, this simple sharing is found to be insufficient to significantly reduce the volume and the weight of the payload necessary for the altimetry mission.